To reduce temperature drift in an analog circuit, a temperature dependent bias current I(T) may be used. Such bias currents are sometimes expressed as a current (I) function of temperature (T), I(T). The bias current I(T) may be generated from a PTAT (Proportional To Absolute Temperature) current-DAC (Digital to Analog Converter) connected to a CTAT (Complementary To Absolute Temperature) current-DAC. The CTAT current is subtracted from the PTAT current, or vice versa, to generate the desired bias current I(T). The resulting I(T) is injected into a sensitive node of the circuit to be compensated.
Temperature sensitive circuits may have both positive as well as negative residual temperature coefficients that need to be trimmed for high drift performance. Note that for a differential stage as in an operational amplifier input stage there are two “opposite” nodes which can be used to inject the temperature dependent current I(T) (i.e., drain node of each of the two input transistors, “injection nodes”). In such a differential circuit, it may be sufficient to generate one bias current I(T), for example with positive temperature coefficient, and to inject the one bias current I(T) into either of the two “injection nodes”, depending on which type of compensation is desired. However in analog circuits having only one input node or in other circuits for some reason having an architecture offering only one “injection node”, two bias currents I(T) have to be provided by the PTAT/CTAT current DAC to provide compensation for both positive and negative temperature coefficients.
If bias current I(T) is established so that I(T)=0 at an elevated temperature above a nominal or target operating temperature for a device, then bias current I(T) always flows in the same direction within a wide interval of operational temperature range. This design approach permits using one simple current mirror to change the sign of bias current I(T) current. Such a sign change may be required because a positive temperature coefficient in bias current I(T) is used to compensate a circuit having a negative temperature dependence, and a negative temperature coefficient in bias current I(T) is used to compensate circuits with a positive temperature dependence. As a result, prior art practice has heretofore provided for specifying the operation or performance of a circuit up to an elevated temperature, such as by way of example and not by way of limitation, 90° C.
However, this widely used prior art approach has disadvantages. At the normal operating temperature, a nonzero current (i.e., bias current I(T)≠0) is injected into a sensitive node. As a result, noise is increased. Further, good compensation for temperature dependence by the circuit can be guaranteed only up to the temperature where bias current I(T)=0. Above to the temperature where bias current I(T)=0 the current mirror used for temperature coefficient inversion may no longer work. By way of example and not by way of limitation, in a configuration of such a current mirror employing a PMOS mirror, the mirror cannot sink any current. Still further, the first trim action when preparing the circuit must be performed at the temperature where bias current I(T)=0. For this reason, a high temperature final package trim must be performed before the operating temperature test. This procedure does not allow easy implementation of graded units that have undergone a room temperature test only. That is, low price units that involve one test at operating temperature with standard trimming cannot be produced using the above procedure. Such a procedure—one test at operating temperature with standard trimming, followed by a test at high temperature to compensate residual drift, followed by another test at operating temperature to check initial accuracy—raises the price of a circuit well above a one-test-required part.
To be able to compensate both negative and positive temperature drifts, currents with both positive and negative temperature coefficients must be generated. Generating a biasing current to represent both positive and negative temperature coefficients is particularly difficult if the biasing current approaches zero and changes sign within the temperature operating range of the device for which compensation is being provided. In such a case involving sign changes of biasing current within an operating range, the entire circuit for generation of the I(T) biasing current has to be duplicated in order to ensure accurate generation of positive-correcting as well as negative-correcting biasing currents.
There is a need for an apparatus and method for compensating change in a temperature associated with a host device that injects substantially zero correction or compensating current into the host device at the nominal operating temperature of the host device.
There is a need for an apparatus and method for compensating change in a temperature associated with a host device that can invert or compensate for temperature dependency substantially across an entire operating temperature range for the host device.